Two-stage soda-oxygen pulping

ABSTRACT

SUBJECTING SOFTWOOD CHIPS TO A MILD DIGESTION WITH SODIUM HYDROXIDE ALONE, DEFIBERIZING THE THUS TREATED MATERIAL AND THEN SUBJECTING THE DEFIBERIZED MATERIAL TO A SECOND DISGESTION WITH SODIUM HYDROXIDE IN THE PRESENCE OF AN EXCESS OF OXYGEN.

United States Patent O 3,691,008 TWO-STAGE SODA-OXYGEN PULPING Hans Edmund Worster, Richmond, British Columbia, and

Marian Franciszek Pudek, Coquitlam, British Columbia, Canada, assignors to MacMillan Bloedel Limited, Vancouver, British Columbia, Canada No Drawing. Filed Apr. 13, 1970, Ser. No. 28,005 Int. Cl. D21!) 1/16 US. Cl. 162-25 9 Claims ABSTRACT OF THE DISCLOSURE Subjecting softwood chips to a mild digestion with sodium hydroxide alone, defiberizing the thus treated material and then subjecting the defiberized material to a second digestion with sodium hydroxide in the presence of an excess of oxygen.

BACKGROUND OF THE INVENTION (1) Field of the invention This invention relates to the manufacture of wood pulp and particularly to a two-stage soda pulping process in which the second stage is carried out in the presence of oxygen.

(2) Description of the prior art It has been well established that sodium sulfide used in kraft pulping is responsible for air pollution with volatile sulfur compounds, such as hydrogen sulfide, methyl mercaptan, dimethyl sulfide and dimethyldisulfide. These compounds are emitted from the digester, spent liquor evaporators and the recovery furnace of a pulp mill. Similarly, in sulfide pulping, air pollution with sulfur dioxide and possibly also sulfur trioxide from sulfur burners, pulping liquor preparation systems, digesters and chemical recovery equipment is commonplace.

It has also been well known that soda pulping does not cause air pollution with volatile sulfur compounds because sodium hydroxide is the only pulping chemical used in this process. However, soda pulping has serious disadvantages compared to kraft pulping, such as lower pulp yield and quality and slower pulping to a given degree of delignificatio-n. Higher chemical applications, longer reaction times and/or higher temperatures can, of course, compensate for a certain extent for the slower pulping of the soda process. However, it has not been possible to improve soda pulp yield and quality by adjustment of process parameters. Chemicals which stabilize the carbohydrates against alkaline degradation, especially peeling, would be required if the pulp yield were to be increased to the level of the kraft process. These chemicals, e.g. borohydride, polysulfide, are much too expensive for use in a pulping process and would also introduce foreign elements (boron, sulfur) into the soda system.

Alkaline pulping in the presence of gaseous oxygen has been proposed in the past. However, it has never been a success commercially because of certain serious shortcomings. For instance, Marshall, US. Pat. 2,686,120, issued Aug. 10, 1954, describes, single-stage alkaline pulping of lignocellulose in the presence of oxygen to produce pulp, vanillin and other oxidation products of lignin substances. However, the pulps obtained by that process are extremely low in tear factor indicating soda pulp quality. While no data on pulp yield obtained from Wood chips is given in the above patent, it will be evident that it must be low due to cellulose degradation.

A process for pulping of bagasse, an annual plant, with ammonia and oxygen is described in US. Patent 3,274,049 but the ammonia-oxygen process clearly would not be suitable for pulping of wood chips.

It has also been known to treat alkaline pulps with oxygen for the purposes of delignification and bleaching. For instance, U.S. Pat. No. 3,384,533 describes the effects of the action of oxygen on pulp in an alkaline medium in the presence of a catalyst, this catalyst being selected from barium carbonate, calcium carbonate, magnesium carbonate, zinc carbonate, alkali metal borates and titanium dioxide. However, none of the examples of this patent describes the treatment of a soda pulp and, moreover the introduction of the above catalysts into the system complicates the delignification, bleaching and recovery processes and significantly adds to production costs.

Yet another process for treating wood pulp with oxygen is described in US. Pat. No. 3,423,282 but in this instance the oxygen treatment is being used as a part of a two-stage oxygen-chlorine treatment and is primarily a bleaching method rather than being directly related to pulping.

It is, therefore, the purpose of the present invention to provide a commercially acceptable soda-oxygen pulping process with pulp yield and quality comparable to those of kraft pulp.

SUMMARY OF THE INVENTION According to this invention, it has been found that soda pulp can be obtained in excellent yield and quality if ligno-cellulosic material, such as wood chips, is subjected to a two-stage soda-pulping process with the second stage being conducted in the presence of an excess of oxygen. The ligno-cellulosic material is first given a mild treatment under heat and pressure with sodium hydroxide only. After this initial mild soda pulping, the treated material is defiberized and the defiberized material is then again treated under heat and pressure with sodium hydroxide, this second treatment being in the presence of an excess of oxygen.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The first stage digestion is sufficient to remove some lignin and extractives and is preferably conducted at a temperature in the range of 140 to 190 C. and a sodium hydroxide application of 10% to 18% by weight expressed as sodium oxide based on the dry weight of the wood chips. After completion of the first stage digestion, the free liquor is separated from the treated chips and can be recycled for re-use and/or recovered for sodium hydroxide regeneration.

The treated chips obtained from the first stage digestion are defiberized, eg in a disc refiner, and are then further pulped in a second stage with sodium hydroxide and a large excess of oxygen under heat and pressure. This second stage digestion is preferably conducted at a temperature in the range of to 130 C. and a sodium hydroxide application of 5% to 12% expressed as sodium oxide based on the dry weight of the defiberized material. The oxygen can be injected into the digester as soon as the alkaline solution is added during the come-up period of the digester to maximum temperature or even after the maximum temperature has been reached. An oxygen partial pressure of to 200 p.s.i.g. is found to be preferable.

With this process it has been found that excellent yield and quality can be obtained without the necessity of any catalyst or cellulose stabilizer in the second stage. It has also been found that the addition of a small quantity of an earth metal hydroxide, eg 1% calcium hydroxide, significantly reduces the requirement of the more expensive sodium hydroxide in the second stage of the pulping process.

The spent liquor from the second-stage digestion can be recovered either together with spent liquor of the first stage or separately in order to regenerate sodium hydroxide by well established methods. When the earth metal hydroxide is used, residual hydroxide or its insoluble reaction products, can be separated from the spent liquor by settling and may then be regenerated for re-use. Separation of any alkali insoluble material is also very desirable in order to prevent operating problems, such as scaling of evaporator tubes and excessive dregs formation on dissolving the smelt.

The process of this invention provides slightly higher unbleached pulp yields, significantly higher unbleached pulp brightness, easier beating and bleachability than kraft pulping. Moreover, the strength properties of the pulp obtained according to this invention are similar to those of conventional kraft pulp.

In addition to the above mentioned advantages, the alkaline pulping method provided by the present invention offers the following advantages over conventional kraft pulping:

(1) It does not cause air pollution with volatile sulfur compounds because of the complete absence of sulfur in the digestion system.

(2) Our process requires no black liquor oxidation and no equipment for collection and/ or combustion of noncondensable sulfur gases.

(3) The process can tolerate direct contact evaporation of spent liquor because of the absence of sulfur.

(4) Our process does not require a high stack for the recovery furnace.

(5) It makes possible the utilization of stack gas heat because of the absence of corrosive gases, such as sulfur or hydrogen sulfide.

(6) Our process causes less corrosion than kraft pulping liquor and makes possible the use of materials other than stainless steel for the digester.

It will, therefore, be evident that our new process is applicable to existing mills or entirely new mills and is especially attractive in areas where kraft pulp mills are at present not permitted because of the problems of air pollution.

The present invention is illustrated by the following non-limitative examples:

EXAMPLE 1 Hemlock (softwood) chips were pulped in a pressure reactor by the conventional kraft process under the following conditions:

The following results were obtained:

Screened pulp yield, percent on dry wood 43.0 Screen rejects, percent on dry wood 0.6 Permanganate number (40 ml.) 19.2 Pulp brightness (Elrepho) 23.8

The pulp was beaten in a P.F.I. mill and handsheets for physical testing were prepared. The results interpolated at 500 and 300 ml. Canadian Standard Freeness were as follow:

Freeness, ml. 0.8 500 300 Beating revolutions, N X 4. 5 6. 95 Mullen. 188 195 197 1. 8

en h, 10.2 10.7

Apparent specific gravity, gm 0.67 0. 70 Brightness 21.6 21. 6

The kraft pulp was bleached by the sequence chlorinationcaustic extraction-hypochlorite-chlorine dioxide-caustic extraction-chlorine dioxide (C-E-H-D-E-D). The yield after this sequence was 39.8% based on dry wood. The brightness increased as follows:

After C-E-H 74.3 After C-E-H-D 85.5 After C-E-H-D-E-D 90.0

A standard pulp aging test gave a brightness loss of 12.4 points after C-E-H-D-E-D bleaching.

EXAMPLE 2 were obtained:

Yield, percent on dry wood 57.2 Kappa number 146 The defibered chips were then split into four equal parts (A, B, C and D) and treated in a pressure reactor separately with oxygen, sodium hydroxide and in some cases earth metal alkalies under the following conditions:

Trials A B O D Type of additive MgCO Mg(OH)z C8.(OH)2 None Quantity of additive, percent based on dry pulp 1 1 1 0 Partial oxygen pressure at 20 C., p.s.i.g 160 160 100 160 Time to 105 0., min 80 80 80 Time at 105 0., min 90 90 90 Consistency, percent 3 3 3 3 Sodium hydroxide, percent on dry pulp, as Na O 9. 8 8.8 7.1 8. 8

The following results were obtained:

Pulp A B C D Yield, percent on dry wood. 1 1. 4 44.1 44. 6 44. 3 Permangauate number (40 ml. test) 18. 4 18. 8 19. 7 19. 5 Brightness 30. 8 30. 5 20. 1 30.5

The pulps were beaten in a PFI-mill, handsheets were formed and tested for properties. The following data were established for 500 and 300 ml. C.S.F.:

Pulp A B C D Freeness, ml. 0.8. 500 300 500 300 500 300 500 300 Beating revolutions,

NX10 2. 35 3. 7 2.30 3. 65 1. 3. 35 2.15 3. 5 195 222 192 211 179 179 208 T 166 158 163 154 164 156 158 141 Breaking length, klll 9. 8 11.0 9.9 10.9 9. 8 10. 3 9. 8 10. 4 Brightness 24. 9 22. 5 25. 5 25.0 23. 5 23. 5 26.1 25. 4

Pulp D was bleached by the sequence chlorinationcaustic extraction-hypochlorite-chlorine dioxide-caustic extraction-chlorine dioxide (C-E-H-D-E-D). The pulp yield after this sequence was 39.2% based on dry wood. The brightness increased as follows:

After C-E-H 81.6 After C-E-H-D 88.3 After C-E-H-D-E-D 91.8

A standard aging test gave a brightness loss of 12 points after C-E-H-D-ED bleaching.

It is to be understood that the invention is not to be limited to the exact details of operation or the exact processes shown and described, as obvious modifications and equivalents will be apparent to those skilled in the art and the invention is to be limited only by the scope of the appended claims.

We claim:

1. An alkaline pulping process which comprises subjecting softwood chips to digestion with sodium hydroxide alone, defiberizing the thus treated material and then subjecting the defiberized material to a second digestion with sodium hydroxide in the presence of an excess of oxygen.

2. A process according to claim 1 wherein the first digestion is conducted at a temperature in the range of 140 to 190 C.

3. A process according to claim 2 wherein the first digestion is conducted at a sodium hydroxide application of to 18% by weight expressed as sodium oxide based on the dry weight of the wood chips.

4. A process according to claim 3 wherein the second digestion is conducted at a temperature in the range of 95 to 130 C.

5. A process according to claim 4 wherein the second digestion is conducted at a sodium hydroxide application of 5% to 12% by weight expressed as sodium oxide based on the dry weight of wood chips.

6. A process according to claim 1 wherein the oxygen in the second digestion is applied at an oxygen partial pressure of 100 to 200 p.s.i.g.

7. A process according to claim 1 wherein during the 6 second digestion some of the sodium hydroxide is replaced by a small amount of an earth metal hydroxide.

8. A process according to claim 7 wherein about 1% by weight calcium hydroxide is used as the earth metal hydroxide.

9. A process according to claim 1 wherein after the first digestion, the free liquor is separated from the treated chips and is recycled for re-use or recovered for sodium hydroxide regeneration.

References Cited UNITED STATES PATENTS 2,913,362 11/1959 Cusi 162-55 3,384,533 5/1968 Robert et al. 162-65 2,904,460 9/1959 Nolan 162-25 3,013,932 12/1961 Hinrichs et al. 162-90 X 2,164,040 1/ 1939 Olfermanns 162-25 S. LEON BASHORE, Primary Examiner A. L. CORBIN, Assistant Examiner US. Cl. X.R. 162-65 

